There are, of course, in the prior art a number of patents to interferometers and a number of patents showing the coating of optical surfaces. Typical examples of interferometers are disclosed in U.S. Pat. Nos. to Wilczynski et al No. 3,359,852; to Jaerisch et al No. 3,858,981; and to Kimura et al No. 3,891,320. Examples of coatings on optical surfaces are shown in the U.S. Pat. Nos. to Gelber No. 3,070,097; to Thelen No. 3,914,023; to Apfel et al No. 3,649,359 and to Negishi et al No. 4,004,851. Furthermore, the patent to Dobler et al U.S. Pat. No. 3,984,581 shows a method of applying reflection coatings on optical elements. However, the coatings used in the foregoing prior art are for antireflection purposes and none teaches the use of a coating of or an optical flat on an interferometer to provide high contrast interference fringes for interferometer measurements.